1. FIELD OF THE INVENTION
This invention relates to a laser radar (or ladar) system having improved transmitter frequency stability.
2. DESCRIPTION OF THE PRIOR ART
Laser radar systems using the homodyne or heterodyne form of signal detection for improved sensitivity generally utilize a main pulse laser which hits a target and is reflected. The reflected signal is received and is mixed with the laser beam of a local oscillator laser in a crystal detector. The transmitting laser must operate at an optical frequency exactly equal to that of the local oscillator (LO) for homodyne detection or different from it by a fixed intermediate frequency (IF) for heterodyne detection.
In a continuous wave laser radar, stability is often achieved by mixing a portion of the transmitted output signal with a portion of the local oscillator output signal to obtain a difference frequency signal. This signal is compared with the required intermediate frequency and any difference is used to control the frequency of the local oscillator or of the transmitter by a servomechanism which acts, by example, to change the length of the optical cavity until the difference frequency is equal to the intermediate frequency.
However, where a pulsed laser is used as the transmitter (for determination of target range), it is extremely difficult to cause it to oscillate at exactly the same frequency on each pulse even if the transmitter laser is forced to oscillate on a single longitudinal mode of its optical cavity. This difficulty exists because changes in the refractive index of the lasing medium, mechanical shock when the laser pulses an output signal, and vibrations of the apparatus imparted by the environment act to alter the effective length of the optical cavity, and therefore its resonant frequency.
Thus it has been virtually impossible to ensure that successive transmitter pulses will always be at the same frequency difference from the local oscillator, which itself may not be completely stable in frequency.